The invention concerns a high-frequency weldable and low-temperature flexible polymer mixture, a sheet or molded piece containing said polymer mixture, and the use thereof.
PVC products have been used for technical applications for many years in the widest possible variety of products. Because of the trend toward reduction of chlorine-containing materials in all areas of life, there is an urgent need for halogen-free alternative products in the construction, automotive, and cable industries. Although EPDM rubbers constitute possible alternatives, they are frequently unsuitable due to cost considerations. For example, WO 93/19118 and WO 89/06256 describe halogen-free formulations that are suitable as cable insulating materials.
A frequent problem in the search for suitable halogen-free alternative products is the fact that while PVC sheets show quite favorable high-frequency welding properties, possible PVC substitute materials such as polyurethane sheets or thermoplastic polyolefin sheets known on the market cannot be welded.
Another property that plays a role in the automobile industry in particular is sufficient low-temperature flexibility of the material. A high degree of low-temperature flexibility is particularly important in cases where dynamic loads are imposed on materials at low temperatures. For example, polyolefin sheet materials are used as decorative materials for concealed air bag systems. In order for a material to be suitable for such an application, it must not become brittle at test temperatures (ordinarily xe2x88x9230xc2x0 C.) or show high tensile strength, which could prevent the air bag lid from penetrating the decorative material. One must also ensure that the material does not shatter because of its brittleness since the resulting fragments could injure vehicle occupants. Brittleness of the material must therefore be avoided, and one must ensure that the air bag opens as desired at low temperatures (without flying particles).
A high degree of low-temperature flexibility is also important for a material used in covers for cargo areas, for example, which may be in the form of sheet-type or rolling covers which must roll up and down without problems at cold temperatures.
At the same time, thermal stability of the material at a temperature of at least 100xc2x0 C. is a requirement for applications in automobiles or other motor vehicles. In this case, thermal stability means that the surface structure or grain of the material must remain intact after storage for several days at the test temperature without showing any increased gloss. Moreover, static elongation at this temperature must be low.
There have been several proposals in prior art for meeting the above requirements:
For example, EP 998-528 discloses non-flammable halogen-free polymer compositions composed of a mixture of ethylene-vinyl acetate-carbon monoxide terpolymer with two additional components selected from ethylene vinyl acetate or polyethylene and an inorganic filler that must show favorable heat resistance.
EP 0-703-271-A1 describes a halogen-free thermoplastic polyolefin composition that is flexible at room temperature. The materials used in this composition include ethylene vinyl acetate copolymers and extremely low-density polyethylene. Although these formulations are often useful for many applications, they have the drawback of showing insufficient heat resistance when exposed to temperatures  greater than 80xc2x0 C.
Moreover, EP 0-977-807 discloses a halogen-free polymer mixture that is flexible at room temperature and shows greater thermal stability than the invention of EP 0-703-271. However, these two compositions do not provide a favorable combination of low-temperature flexibility and sufficiently high thermal stability.
It is known that sufficient low-temperature flexibility can be achieved for a material by including correspondingly soft components. But as this results in insufficient thermal stability, no satisfactory results have yet been obtained by this method. There are currently no solutions that provide a high-frequency weldable polyolefin sheet as a substitute for a PVC sheet, e.g. for the applications mentioned above.
The purpose of the invention is therefore to provide a halogen-free polymer mixture that does not have the drawbacks described above in order to provide a sufficient degree of high-frequency weldability and favorable weld seam strength with simultaneous high low-temperature flexibility and favorable thermal stability.
The invention achieves this purpose by forming a high-frequency weldable and low-temperature flexible polymer mixture, containing
a) approximately 1 to 30% by weight of a copolymer with ethylene and a polar comonomer selected from the group of vinyl esters of saturated C2-C8 carboxylic acids, C1-C12 alkyl acrylates, and/or methacrylates, with the copolymer having an MFI (melt flow index) (according to DIN 53,735; 190xc2x0 C., 2.16 kg) of approximately 0.1 to 30 g/10 min, and preferably approximately 0.5 to 10 g/10 min, and a comonomer content of less than approximately 25% by weight, and preferably less than approximately 19% by weight;
b) approximately 1 to 40% by weight of components b1) and/or b2), selected from:
b1) a terpolymer, with one monomer each being selected from one of the following three groups:
(1) ethylene, propylene, and/or butylene,
(2) the group of vinyl esters of saturated C2-C8 carboxylic acids, C1-C12 alkyl acrylates, and/or methacrylates, and
(3) carbon monoxide and/or glycidyl acrylate or methacrylate,
xe2x80x83with the content of monomer (1) being approximately 30 to 90% by weight, the content of monomer (2) being approximately 10 to 70% by weight, and the content of monomer (3) being approximately 1 to 20% by weight, and the MFI (190xc2x0 C., 2.16 kg) of the terpolymer being between approximately 0.1 and 30 g/10 min, or
b2) a copolymer, with one monomer each being selected from one of the following two groups:
(1) ethylene, propylene, and/or butylene, and
(2) the group of vinyl esters of saturated C2-C8 carboxylic acids, C1-C12 alkyl acrylates, and/or methacrylates;
xe2x80x83with an MFI (190xc2x0 C., 2.16 kg) of approximately 0.1 to 30 g/10 min and a comonomer content of more than approximately 25% by weight, and
c) approximately 35 to 75% by weight of a thermoplastic polyolefin with a melting point above approximately 115xc2x0 C., selected from the group of ethylene homopolymers, ethylene copolymers, propylene homopolymers, and propylene copolymers or mixtures thereof, with this polymer being optionally grafted with approximately 0.1 to 7% by weight, preferably approximately 0.5 to 5% by weight, and particularly preferably approximately 0.05 to 3% by weight of an unsaturated carboxylic acid, unsaturated dicarboxylic acid, or esters and/or anhydrides thereof,
with the amounts of a) through c) totaling 100% by weight, and the percentage of components a)+b1) or a)+b2), with respect to the total amount of the polymer mixture, being 50% by weight or less. The lower limit for the percentage of components a) +b1) or a) +b2) is approximately 25% by weight, as high-frequency weldability might otherwise be impaired.
The invention therefore provides a polymer mixture in the form of a composition tailored to the desired application that essentially comprises three components and shows the required set of characteristics. On the one hand, polar polymers are included that enable high-frequency weldability, support increased low-temperature flexibility, and provide a high degree of static elongation. These polymers containing polar groups are used in a maximum amount of 50% by weight. Surprisingly, the above polymer mixture also provides increased temperature stability, but without impairing weldability and outstanding low-temperature flexibility, and sufficiently low static elongation at elevated temperature is also ensured.